A spectral sensitivity of a solid-state imaging sensing device such as a CCD or a CMOS used in a digital still camera or a video camera is featured in having a high sensitivity to light in a near-infrared region, compared with a visibility characteristic of a human being. Accordingly, in general, a visibility correction filter is used in order to adjust a spectral sensitivity of such solid-state imaging sensing devices to the visibility characteristic of the human being.
As such a visibility correction filter, Patent Reference 1 (JP-A H06-16451) discloses a near-infrared cut filter glass whose spectral characteristic is adjusted by making Cu2+ ions exist in a glass such as a fluorophosphate glass or a phosphate glass.
Further, there is known a near-infrared cut filter having an excellent characteristic, which is provided with an optical multilayer film where a plurality of high-refractive index layers and low-refractive index layers are alternately stacked, on a surface of the near-infrared cut filter glass as above, in order to accurately determine and sharpen a transmitting wavelength region, to thereby transmit a wavelength (400 to 600 nm) of a visible region efficiently and also cut a wavelength (700 nm) of the near-infrared region sharply (for example, see Patent Reference 2 (JP-A H02-213803)). Other than the above, there is a case where an anti-reflection film is provided on a surface of a near-infrared cut filter glass, for the purpose of suppressing reflection of a glass substrate surface to improve a transmittance.
In a case of a near-infrared cut filter, the optical multilayer film is a film in which high-refractive index layers made of titanium oxide, tantalum oxide, niobium oxide or the like and low-refractive index layers made of silicon oxide or the like are alternately stacked on a glass substrate, for example, and, by setting composing materials, thicknesses, a layer number, or the like of the high-refractive index layer and the low-refractive index layer appropriately, selectively transmits light by using interference of light.
Besides, as an optical apparatus and an imaging apparatus become smaller and thinner, an optical filter such as a near-infrared cut filter to be used is also desired to be thinner in thickness in an optical axis direction. With respect to thinning of a substrate, there was a problem that providing an optical multilayer film on a substrate whose plate thickness is small causes distortion (warpage) of a substrate shape noticeably. To cope with such a problem, there is proposed a method of suppressing warpage of a substrate by setting a ratio of the number of layers of optical multilayer films formed on both front and rear surfaces of the substrate to a predetermined range (over 1 and equal to or less than 2.5) (for example, see Patent Reference 3 (JP-A 2006-220873)).